The present invention relates noise canceling, and more particularly to noise canceling using a single return conductor. Even more particularly, the present invention relates to noise cancelling using a single return conductor for cancelling noise in multiple signal carrying conductors.
Many systems that employ a full duplex signal path, i.e., a signal path in which signals can be transmitted and received simultaneously between two or more circuit modules, or units, utilize complex and costly approaches to canceling noise introduced as a result of differences in ground potential at each of the two circuit modules. Such methods commonly include transmitting and receiving signals differentially, i.e., on respective in-phase and out-of-phase carriers, or by transmitting and receiving a single-ended signal and subtracting differences in ground reference from the signal at the receiving end. Each of these commonly used approaches requires that at least two conductors be used for each of a forward and a reverse signal path. (In the case of transmitting and receiving differentially, one conductor is used for the in-phase carrier and another is used for the out-of-phase carrier. In the case of the single-ended signal from which is subtracted the differences in ground potential, one conductor is used to transmit the single-ended signal, and another conductor carries the ground reference of the unit transmitting the single-ended signal to the unit receiving the single-ended signal.) This necessitates the use of at least four conductors for full duplex operation, and if multiple forward and reverse signal paths are utilized, two conductors may need to be used for each forward and reverse signal path.
For example, in FIG. 1, a schematic diagram is shown of a prior art system for noise canceling, wherein multiple noise canceling return conductors are used to carry ground reference signals for noise canceling in multiple signal carrying conductors.
Shown are a first module 10 (or first unit) and a second module 12 (or second unit) coupled together by a full duplex signal path 14 made up of four individual conductors 16, 18, 20, 22. The particular nature of the first and second units 10, 12 depends upon the particular application to which noise cancelling is applied. For example, noise cancelling may be used when the full duplex signal path 14 is used to carry audio signals between the first and second units 10, 12.
For purposes of this description the first two of the conductors 16, 18 are referred to as a forward signal path, and the second two of these conductors 20, 22 are referred to a reverse signal path.
Within the first unit 10, an output signal is passed through an amplifier 24 and onto a first of the conductors 16. A second of the conductors 18 is coupled to a ground reference 25 within the first unit 10. Within the second unit 12, the first and second conductors 16, 18 are coupled to a adder 26, with the first conductor 16 being coupled to a non-inverting input of the adder 26, and with the second of the conductors 18 being coupled to an inverting input of the adder 26. The sum of the signals on the first conductor 16 and the second conductor 18, as determined by the adder 26, is a noise canceled output 27, which may be used by the second unit 12 in numerous ways, depending on the particular application for which the units 10, 12 are suited.
In a similar manner, a noise canceled input 28 is provided to the first unit 10 from another adder 29. The third conductor 20 provides an amplified input signal to a non-inverting input of the adder 29. The amplified input signal is provided to the third conductor 20 by an amplifier 30, which receives an input signal from the second unit 12 and generates the amplified input signal in response thereto. The fourth conductor 22 is coupled to a ground reference 31 within the second unit 12 and is coupled to an inverting input of the adder 29.
As a result of this configuration, noise introduced into the third or fourth conductor 20, 22 is subtracted out of the amplified input signal by the adder 29 in order to create the noise canceled signal at the noise canceled input 29. The noise canceled input signal can be used by the first unit 10 in a manner consistent with the particular application for which the units 10, 12 are suited.
Disadvantageously, the above described prior art system requires that two conductors utilized for each of the forward signal path and the reverse signal path, thus necessitating a total of four wires for the full duplex signal path. When multiple forward signal paths and/or multiple reverse signal paths are to be employed, two conductors may, in some applications, need to be utilized for each of the forward signal paths and/or reverse signal paths. Similarly, when signals are transmitted and/or received differentially, i.e., modulated onto in phase and out of phase carriers, two conductors must be utilized for each forward signal path and for each reverse signal path. Thus, what is needed is a system that provides for noise cancellation using fewer than two conductors for each signal path, and preferably using only a single return conductor for multiple forward and reverse signal carrying conductors.
The present invention advantageously addresses the above and other needs.